It has previously been proposed to produce a printing plate by selectively collapsing the open cell structure of a thermoplastic plate to provide relief (depression of non-printing areas), and thereby to define the non-depressed portions necessary for performing a printing operation.
It is an object of this invention to carry out the foregoing basic method in a more effective manner and at lower cost.
It is a further object of the invention to achieve a more complete collapse of the cell structure in the areas where relief is desired, and to better define the planar difference between the raised and relief portions of the plate.
This invention relates to the method of making a printing plate from a plate of thermoplastic material having an open cell structure. A suitable coating material is brought into contact with one surface of the thermoplastic plate. The coating is then caused to adhere to the plate in accordance with the configuration of an area of the surface of the plate corresponding to the image. The coating has different thermal absorption from the plate so that the portion of the plate to which the coating has adhered will absorb infrared rays more readily than other areas of the plate. The remaining coating material which is not adhered to the plate is removed.
The plate is next heated to a temperature near to, but below, the temperature at which the plate structure becomes semisolid.
Infrared rays are then directed upon the surface of the plate carrying the coating, and since these rays are more readilly absorbed by the coating than by the plate, the cells of the plate collapse under the coating.
The aforesaid heating step which brought the plate to a temperature near to, but below, the temperature at which the plate became semisolid enables the application of infrared rays to collapse the cells in shorter time than would otherwise be required, resulting in improved fidelity of the plate.
In addition cooling fluid is passed through the innerconnected cell structure of the plate during said infrared heating step to cool the portions of the plate where collapse of the cell is not desired.